clearer url rule slash error

Author: TheGreatAlgo

CHANGES.rst

@@ -11,6 +11,7 @@ Unreleased
 -   ``_plain_int`` and ``_plain_float`` strip whitespace before type
     enforcement. :issue:`2734`
 -   Fix empty file streaming when testing. :issue:`2740`
+-   Clearer error message when URL rule does not start with slash. :pr:`2750`
 
 
 Version 2.3.6

src/werkzeug/routing/rules.py

@@ -454,7 +454,8 @@ def __init__(
         websocket: bool = False,
     ) -> None:
         if not string.startswith("/"):
-            raise ValueError("urls must start with a leading slash:", string)
+            raise ValueError(f"URL rule '{string}' must start with a slash.")
+
         self.rule = string
         self.is_leaf = not string.endswith("/")
         self.is_branch = string.endswith("/")